The present invention relates to a preparation process of an oil extended rubber for a tire, the oil extended rubber for a tire, and a rubber composition and tire which use the oil extended rubber.
Conventionally, in processing a rubber for a tire, mastication has been carried out before carrying out kneading of materials in order to improve processability of rubber or lower a viscosity of rubber. Particularly, processability of a natural rubber (NR) having a large molecular weight and unstable viscosity varies depending on a degree of mastication, and dispersibility of chemicals can be improved, therefore, mastication step has been an essential step.
In view of such a background, for the purposes of lowering a high viscosity of a rubber to make kneading easy, and further, improving dispersibility of other components to be blended, a rubber containing an oil (oil extended rubber) is commercially available.
The commercially available oil extended rubber is mainly a synthetic rubber, and for the purpose of improving processability, such a rubber obtained by homogeneously mixing an oil with the synthetic rubber to be baled is generally used.
However, in recent years, environmental problems have been regarded as important, and regulations on CO2 emission have been made more rigorous. Further, since there is a limitation on petroleum resources, and supply thereof has been decreasing year by year, oil price is predicted to surge more in future, accordingly, there is a possibility that use of raw materials derived from petroleum resources such as a synthetic rubber has its limit. Therefore, assuming that petroleum oil is depleted in the future, it is desirable to use resources other than petroleum such as NR and epoxidized natural rubber (ENR).
As for a process for preparing an oil extended rubber by mixing an oil in a rubber, various processes are known and the preparation process is not particularly limited, and for example, a process of adding an oil to a solid rubber in a kneader is known. As for a rubber obtained by solution polymerization, a process of adding an oil to a polymerized rubber containing a solvent and then removing the solvent is known. As for a rubber obtained by emulsion polymerization, a process of adding an oil directly to a rubber in a state of a latex, and forcibly stirring the mixture and then coagulating the rubber is known.
In the case of kneading an oil using a kneader after solidifying a rubber, the rubber easily slips inside of the kneader if the oil is charged together with the rubber, therefore only a rubber is kneaded at first, and then an oil is added thereto to be kneaded. However, in this case, since a viscosity of a rubber is high, shearing strength is easily applied on the rubber during a period of time until the oil is added, and thus a rubber molecule is partially broken. When a rubber product is prepared using an oil extended rubber obtained in this manner, since properties of the rubber product such as abrasion resistance, strength, and flex crack growth resistance are deteriorated, there has been a problem that such a rubber product is lowered in strength and abrasion resistance. In addition, in the preparation process as described above, in which an oil is added in a stage of kneading after a step of mastication of a rubber, there has been a problem that it takes time to absorb an oil, and thus, productivity is deteriorated.
Further, when an oil is added to a rubber latex to be stirred, there has been a problem that a degree of dissolving the oil into the rubber latex is low, and thus, the oil easily remains in an aqueous phase. In order to prevent such a problem, a method of stirring a rubber latex and an oil at a high speed and then coagulating the mixture is known. However, in this case, in order to homogeneously mix the oil in the rubber, the coagulated rubber is kneaded with an extruder or the like, which results in applying shearing strength on the rubber, and causing a problem that breakage of a rubber molecule is generated.
JP-A-2005-263956 discloses a rubber composition for a tire tread, in which mal-distribution of silica and compatibility with an oil can be improved by compounding specified amounts of a specific rubber component, specific silica, oils derived from vegetables, a silane coupling agent and an anionic surfactant, and further a rolling resistance property and wet grip performance are improved without lowering processability and abrasion resistance. However, it is not taken into consideration to improve processability and physical properties of a rubber by preparing an oil extended rubber by a specific preparation process.